Rejuvenated chromium plating medium containing chromic compound

ABSTRACT

BRIGHT DECORAVTIVE CHROMIUM PLATE CAN BE OBTAINED FROM CHROMIUM PLATING BATHS CONTAINING A CHROMIC-COMPOUND FOR PLATING WITH SUCH COMPOUND HAVING AT LEAST CARBOXYLIC ACID CONSTITUENTS AND FURTHER EXHIBITING READY WATER SOLUBILITY. REJUVENATION OF SUCH BATHS, WITH A SULFITE COMPONENT, WILL PROVIDE CONTINUED DESIRABLE PLATING PERFORMANCE, PARTICULARLY IN THE LOW CURRENT DENSITY CHROMIUM PLATING RANGE. TYPICALLY, ONLY VERY MINOR AMOUNTS OF SUCH SULFITE COMPONENT, FOR EXAMPLE PROVIDED BY ADDITION TO THE BATH OR BY FORMATION IN SITU, ARE NEEDED TO INSURE EXTENDED HIGH PERFORMANCE OPERATING LIFE OF THE THUS CHEMICALLY REBALANCED TRIVALENT CHROMIUM PLATING BATH.

United States Patent Oflice 3,706,639 Patented Dec. 19, 1972 3,706,639 REJUVENATED CHROMIUM PLATING MEDIUM CONTAINING CHROMIC COMPOUND John Edwin Bride, Mentor, and Francis Huba, Painesville, Ohio, assignors to E. I. du Pont de Nemours and Company, Wilmington, Del.

No Drawing. Filed Feb. 19, 1971, Ser. No. 117,091

- Int. Cl. C23b 5/06 U.S. Cl. 20451 16 Claims ABSTRACT OF THE DISCLOSURE the thus chemically rebalanced trivalent chromium plating bath.

BACKGROUND OF THE INVENTION Decorative chromium plating from baths containing chromium in the trivalent state in association with at least carboxylic acid constituents has offered promise for commercial use; US. Pat. 3,006,823, for example, describes a recently developed aqueous electrolytic plating bath containing a chromium complex of chromic ion and carboxylic acid. However, in working such baths, desirable performance in chromium rate of deposition in the low current density area is not always achieved for an extended period. Undesirable plate may be experienced in the low current density region, not only for plating thickness, but also with regard to the extent or range for which an extendible plate thickness can be obtained.

Formerly, in the electrolytic deposition of chromium from baths containing trivalent chromium compounds, e.g., chromium sulfate, chromium chloride or chromium acetate, which baths were maintained as nearly neutral as possible, such as a pH more or less within the range of 4-6, the production of deleterious high valence chromium could take place at the anode. Toovercome or suppress this formation of higher valence chromium, it was heretofore known to add to the plating bath a reducing agent, or to employ a chromium anode, or a diaphragm cell. For example, US. Pat. 1,922,853 discloses the use of such anodes, or such a cell, or the use of a reducing agent including chromous salts, bisulfite, or the like.

SUMMARY OF THE INVENTION It has now been found that in the plating of decorative chromium plate with baths containing chromium in the trivalent state in association with carboxylic acid constitutents, and which baths are virtually always more acid than a 4-6 pH, that the presence of a sulfite component can enhance the deposition of the plate in a low current density area during extended working of the bath. Such enhancement is exhibited both in acceptable plating thickness and in the range of such thickness within the region. This augmentation in the low current density region is provided by the presence of even very minor amounts of sulfite component.

Moreover, the efiiect obtained 'by the sulfite component will be achieved even though such baths fail to demonstrate any formation or buildup of higher valence chromium. For example, analysis of such baths by the diphenyl carbazide test has always been negative. Moreover the presence of the sulfite component does not provide for deleterious effect on high current density area platingcharacteristics, thus further enhancing the commercial potential of baths containing such component.

In part, the invention is directed to an aqueous electrolytic plating bath for the plating of bright chromium plate, where the bath is maintained at a pH within the range of between about 1.5-4.9, which bath has enhanced electrodeposition of such plate, and comprises a complex, water-soluble chromic compound for the deposition of chromium plate, with the complex containing carboxylic acid constitutents, and the bath containing a sulfite component. The sulfite component provides in the bath at least one compound, where such exists, selected from the group consisting of metal sulfites, metal meta bisulfites, metal bisulfites, trialkylammonium bisulfites, and mixtures of same. Metal for such compounds is selected from the group consisting of alkali metals and alkaline earth metals. I The invention is further directed to the method of chromium plating an article with a decorative chromium plate from a bath showing enhanced deposition in the low current density area and is also directed to the method of enhancing such electrodeposition of bright chromium plate from an aqueous chromium plating bath.

DESCRIPTION OF PREFERRED EMBODIMENTS The chromic compound contains carboxylic acid constituent, e.g., the chromic carboxylate of the aqueous electrolytic plating baths disclosed in U.S. Pat. 3,006,823. As taught therein, the chromic compound is preferably a chromic carboxylate of an alpha-hydroxy carboxylic acid, for example glycolic and latic acids, although other acids can 'be employed such as will be discussed more particularly hereinbelow. The carboxylates may be added to the plating bath as such or, as taught in US. Pat. 3,021,267, they may be provided by dissolving chromic hydroxide or carbonate or even metallic chromium in the carboxylic acid and the pH adjusted with sodium hydroxide or carbonate. As used herein the term chromic carboxylate is meant to refer to such compounds of trivalent chromium and carboxylic acid as are exemplified by such patents.

However, the bath may also contain some to all of a water-soluble chromic compound containing carboxylic acid constituents plus halogen constituents which can be chlorine, fluorine, bromine, iodine, or mixtures thereof. However, in typical commercial plating operation, bromine and iodine are often not used, for economy and to avoid evolution of visible noxious fumes at the anode. Theirefore, chlorine and fluorine are almost exclusively use Although it is not meant that the invention be limited to chromium plating from liquid medium containing a chromic compound having an acid constituent representative of only especial groups of carboxylic acids, such acids which can or have been used for the chromic compounds are typically exemplified by dicarboxylic and monocarboxylic acids, free from carbon-to-carbon unsaturation, and with or without hydroxyl groups. For plating efiiciency and water solubility, advantageously these acids are non-aromatic acids containing less than about 10 carbon atoms; representative acids include glycolic acid, lactic acid, formic acid, oxalic acid, and their mixtures. Preferably, for enhanced plating performance, plus economy, although it is not meant that this invention be bound to baths wherein the chromic compound contains only carboxylic acid constituents alone, the baths used virtually always contain a chromic compound which has carboxylic acid constituents supplied at least in part by glycolic acid. A compound of any of these acids such as a salt or an ester thereof, which acts in any of the reactions, such as those discussed in more detail hereinbelow whereby the complex is formed, in the same manner as the free acid, can be used.

In preparing a plating bath containing a chromic carboxylate, typically chromic acid can be reacted with the carboxylic acid. For example, to reduce one mole of the chromic acid, 0.5 mole of glycolic acid may be used and 1.0 mole of glycolic acid is generally present'to form the desired complex. Typically excess acid, e.g., a 0.1 mole excess, is present to insure reduction and complex formation. Thus, more particularly, chromic acid present as a solution in water, may be slowly added to glycolic acid, also dissolved in water, at 90-100 C. When addition is complete, the solution can be heated to reflux and typically maintained at reflux for about one hour or more to complete reaction.

When halogen is incorporated in the complex, the complex can be prepared by any of several methods. One method is the straightforward combination of chromium metal with carboxylic acid plus hydrochloric acid. When such combination includes particulate chromium metal to reduce reaction time, the reaction can be highly exothermic, and therefore caution needs be taken in carrying out same. Typically for enhanced reaction efficiency, as the reaction proceeds and the evolved heat starts to diminish, external heating is applied; and, where the reaction proceeds in aqueous medium such external heating can involve refluxing of the reaction mixture to augment completion of the reaction.

The complex of this type may also be prepared from the carboxylic acid and hydrochloric acid in admixture with chromic acid, typically charged to the reaction medium as a solution of chromic acid in water. The chomic acid can be supplied by any of the suitable substances for forming chromic acid in water, e.g., chromium trioxide. The reaction resulting from this method is also exothermic and caution in the use of such method is also thus advisable. These complexes may further be prepared by reaction of chromic halide, with such halide corresponding to the halide that is to be present in the complex; the chromic halide is reacted with the carboxylic acid, with this reaction further involving the addition of strong base, e.g., an alkali metal hydroxide. For example, CrF -9H O may be used in this methodand will readily yield a chromium/ carboxylic acid/fluoride complex involving exothermic reaction conditions.

These carboxyl containing complexes virtually always contain a molar ratio of chromium atoms to carboxylate constituent within the range of 1:07 to 1:30. Where halogen is present the complex essentially always has a molar ratio of chromium atoms to halogen atoms within the range of 1:0.1 to 1:3.5. Especially preferred ratios, based upon desirable plating performance and economy can depend upon the acid and also upon the halogen constituent when such is present. Thus for example, for a chromic carboxylate prepared with glycolic acid, the ratio of the chromic ion to glycolic is preferably maintained within the range from about 1:1.1 to 1:2.1. For a complex containing a substantial amount of the glycolic acid for the carboxylate, which complex further contains chloride as the major amount, to all, of the halogen, the ratio of chromium atoms to halogen is preferably within the range of about 1:04 to 1:1. However, when the halogen in such a complex is preponderantly, to all, fluoride, the ratio of chromium atoms to halogen is preferably within the range of 1:2.6 to 1:3.2.

The complex is generally present in the bath in an amount to provide from about 25 to about 150 grams of chromium per liter, that is, the molar concentration of chromium in the plating medium is generally within the range from about 0.5 to about 3.0, although for the baths containing chromic carboxylate as little as 0.1 mole of chromium in the plating medium is servicable. The more 'highly concentrated baths having augmented viscosity are not well suited for deposition of chromium onto a substrate immersed therein. Thus such baths having molar concentration of chromium above about 1.5 are typically used in portable plating devices for spot plating, e.g., brush plating. Regardless of plating method, the bath is one where the complex is present in a liquid medium supplied all, or virtually all, by water. For example, in a bath containing a chromic carboxylic where the carboxylic acid is present in excess and the acid is liquid at normal temperature and pressure, a minor contribution to the medium can come from the excess acid.

The sulfite component of the bath is obtained by providing the bath with at least one compound, where such exists, of a metal sulfite, or metal bisulfite, or metal meta bisulfite, or trialkylammonium bisulfite as well as with mixtures of these. It is recognized that in the bath the water will react with the metal meta bisulfite to produce the corresponding sulfite. The metal portion of the sulfites and the like can be supplied by the alkali metals and the alkaline earth metals with potassium and sodium being preferred for efficiency and economy. These substances may be simply added to the bath or they may be formed in situ for example sodium bisulfite may be provided in the bath by bubbling S0 in the bath along with the addition of sodium hydroxide, or by the addition of trialkyl amines to the bath while S0 is bubbled therein, or the sulfite component may be provided by formation in situ plus addition to the bath.

Enhanced low current density plating coverage may also be obtained by addition to the bath of salts of formaldehyde bisulfite, where such exist, and particularly the alkali metal and alkaline earth metal salts. However, since the beneficial effect is not ostensibly as lasting as with the hereinabove discussed substances for the sulfite component, these salts, e.g., sodium formaldehyde bisulfite, are not the preferred salts for use.

For addition to the bath, the sulfite component can be added in any convenient manner, for example as a water solution or as a blend in solution with chromic compound; such a blend is then added to the bath in part for replenishing the bath during working. The sulfite component is generally not present in the bath when the bath is freshly prepared although such component may be present and if so, is usually added before the freshly prepared bath is electrolyzed. In general the bath need not contain any sulfite until working of the bath is accompanied by some drop off in plating speed in the low current density area. At that time, the sulfite component can be used to enhance the plating speed, but preferably added, for economy, in an amount not substantially greater than about 5 weight percent of the bath. More typically the bath is provided with sulfite in an amount between about 1 to 3 weight percent based on the weight of the bath.

Although particular baths may contain certain specified additional substances and in particular amounts, e.g., the amounts of free carboxylic acids specified for the bath of US. Pat. 3,021,267, in general the bath can also contain a salt of a strong acid preferably, for economy, an alkali metal salt. Such salts enhance the conductivity achieved in the electroplating operation. Most preferably, for economy, the cation of the salt is sodium, potassium or their mixtures, and the strong acid anions should typically be those of an acid having a dissociation constant of at least K: 10- for example, chloride. The plating both usually contains between about 50-200 grams per liter of such salts. The bath can also contain boric acid, or an equivalent to boric acid in aqueous solution, such as borax, boron oxide, or sodium oxy'fluoborate. Such compounds operate in the bath to augment the rate of deposition of the chromium and are typically used in an amount between about 10-70 grams per liter of bath.

Before deposition of chromium, the bath pH is adjusted to within a range depending upon the complex present,

e.g., for a bath containing a major amount of one simple chromic carboxylate complex, the bath is adjusted to a pH within the range from about 1.5-3.0. For a bath wherein the major amount of complex is contributed by such complex further containing halogen atoms, which complex for overall plating efficiency and for an augmented extended bright range is preferred, the bath is preferably adjusted to a pH within the range from about 2.0-3.5. Therefore, for most of the plating baths in the present invention, such baths will be at pH between 1.53.5. However, there can be achieved an acceptable bright chromium plate for such halogen containing complexes, and enhanced deposition from the present invention at the low current density area, when the bath is maintained within the range from about 1.8-4.9. Also, for complex chromic carboxylate baths having blends of chromic carboxylates, as shown in US. Pat. 3,021,267, the bath can be maintained within a pH of between 2.7 and 4.5. Thus, for these particular complexes, such acceptable ranges are used.

For such baths having a halogen containing complex, the most desirable pH range can depend upon the particular make-up of the complex present in the plating medium with, for example, a complex containing a substantial amount of fluorine as the halogen being preferably maintained at a slightly more elevated bath pH than a bath where chlorine supplies the major amount of a halogen. Any adjustment of the bath can be readily carried out with a base, particularly alkali metal carbonates or hydroxide, with sodium or potssium hydroxide or their mixtures being preferred. Before addition to the bath, such material for adjusting the bath pH can be initially dissolved in water and the water solution then added to the bath.

The temperature of the bath during plating, for efiiciency and economy is in part dependent upon the complex present in the bath, with such baths as contain a major amount of chromic carboxylate, being typically maintained during plating at a temperature within the range from room temperature up to about 90 C., but more typically up to about 65 C. For those baths where the major amount of the complex contains a halogen, the temperature is advantageously for enhanced plating performance not substantially above about 50 C.

During plating, the object to be plated is made the cathode, for example, immersed in the plating bath, or the cathode in a spot plating system using a portable plating device supplying the electrolyte and a positive source of electrical current, e.g., brush plating operation, where the plating medium is contained in the brush, and an inert anode is used such as a carbon, graphite, platinum, or platinized titanium anode. Exemplary cathode substrates for receiving the plate include metal such as steel, brass, copper, copper alloys, bronze, zinc die castings, and nickel. Additionally such plating can beperformed on plastic surfaces which are activated or prepared for an electroplating operation.

The plating can be typically carried out in any vessel useful for chromium electroplating such as tanks lined with corresion resistant material including glass, ceramic material, polyvinyl chloride and the like. Also, electrodeposition with such plating baths containing their organic additives can be performed by any conventional plating technique including rotating receptacle coating apparatus immersed in the plating bath. Although diaphragm compartment cells may be employed for plating, they are not preferred for economy. 1

The following examples show ways in which the invention has been practiced but should not be' construed as limiting the invention. Unless otherwise specified, plating tests in the examples are conducted in a modified Hull cell. The standard Hull cell is a trapezoidal box of nonconductive material at the opposite ends of which are positioned anode and cathode plates, as has been more particularly described in U.S. Pat. No. 2,149,344. For either the standard or the modified Hull cell, it is possible to easily determine the elfective plating range of a plating composition under varying conditions. The current density at any point on a cathode is determined according to the formula A-=C(27.7--48.7 log L) wherein A is the current density at the selected point, C is the total current applied to the cell and L is the distance of the selected point from the high current density end of the plate.

In the modified version of the Hull cell used herein, /2- inch holes are introduced inthe parallel sides'of the cell adjacent the anode and cathode whereby, upon immersion of the cell in another -vessel containing plating solution, into which vessel the cell will fit very closely, and also improved electrolyte circulation and consequent improved temperature control is afforded, as more particularly described in an article appearing in Plating, volume 46, No. 3 (1959), page 257.

I Example 1 Into a container there is placed 0.78 mole of chromium metal, 1.8 mole of glycolic acid of 70 percent strength, that is, 70 percent of glycolic acid and a balance of water, and 0.4 mole of 37.3 percent strength of hydrochloric acid which is 37.3 percent by weight HCl in water. The container is covered and good ventilation is provided. After the ingredients are placed together in the container, dissolution of the chromiumstarts slowly but gradually increases thus supplying heatto the reaction period. As the reaction continues the temperature of the reaction medium reaches 77 C. without external heating and the chromium metal can be seen by visual inspection to be substantially dissolved. As the temperature starts to subside from 77 C., external heating is applied and the temperature of the reaction medium is permitted to reach C. after all the chromium is dissolved. Total reaction time, i.e., to complete chromium metal dissolution, is about 6 hours. Thereupon the solutionis heated at reflux, reaching a temperature of 105 C., for about 2 hours, and is thereafter permitted to cool.

The resulting complex has a molar ratio of chromium to glycolic acid of 1:231 and of chromium to chloride of 1:0.513. To prepare a bath for plating the following substances are added to obtain the following concentrations; 55 grams per liter (g./l.) of boric acid, g./l; of potassium chloride and sufiicient 40 percent strength sodium hydroxide, that is, 40 weight percent NaOH and a balance of water, to provide a pH of the bath of about 3. A 1500 milliliter portion of this bath is taken for plating tests and before plating is combined with 0.67 percent by volume of bis(2-methoxy ethyl)ether for maintaining plating deposition at high current density range.

The bath is electrolyzed at a rate of about 20-30 amp hours per gallon and is thereby prepared-forplating in the above-described modified Hull cell. In the Hull cell, graphite rod'anodes are used and the cathode employed is 3 by 2%" brass panels, one panel for each test, each panel being nickel coatedprior to use in the cell. Eachtest is carried out using 10 amperes current for a 3 minute cycle.

These tests are carried out until the bath has been used for a total of 60 amp hours per 1500 milliliter bath. At this point in operation, as shown in the table below, sodium metabisulfite is added to the bath and plating tests are continued. After an additional60 amp hours per 1500 milliliter bath, an additional increment of sodium metabilulfite is added to the bath as shown in the table below. 'After the bath has assimilated this addition, plating tests are continued. The results shown in the table below give the rates of deposition at 7 specific current density levels as well as the total bright range. The chromium thickness reported at each density level is in micro-inches per 3 minute cycle. The current density range shown in the table is in amperes per square foot (a.s.f.). i

The above results demonstrate the loss in performance in the low current density range that can occur during working of thebath. However, as shown in the table, an excellent restoration of the plating performance in the low current density range can be achieved by addition of very minor amounts of sodium metabisulfite. Following such additions, a very short electrolyzing period to assimilate such addition into the bath, will thereafter yield plated panels which in additionto having the augmented chromium thickness in the low current density range will also exhibit an acceptable. plate-acrossfthe' bright range as shown above yielding a highly desirable decorative chromium plate across such range. 1

' Example 2 Into a reaction vessel containing 3000 milliliters water, there is added 4398 grams glycolic acid. ;This mixture is heated to 70 C. and is then gradually combined with .a solution of chromic acid containing 2700 grams chromium, expressed as CrO and 1680 milliliters water. During the slow addition of the chromic acid solution, the temperature in the reaction medium is maintained at 70 C. After addition of the chromic acid solution, the reaction mixture is heated to 90 C. and held at that temperature for one hour. Subsequently the mixture is permitted to cool and is diluted with water to 11,232 milliliters.

A portion of this resulting solution, suflicient to provide a chromium concentration of 52 grams per liter is taken and is blended with the following substances to provide a test bath having a total volume of 1500 milliliters and a concentration of added substances as follows: 150 g./l. potassium chloride, 62 g./l. of H BO and, 86 milliliters perliter glycolic acid of 70 percent strength. The pH of this bath is adjusted in the manner of 'Example 1 to a level as shown in the table below, and the bath is electrolyzed for 20-30 amp hours per gallon. v

Plating tests are then conducted with this bath and are carried out in the manner of Example 1 with the results being reported in the table below. After the initial tests, the results of which are first shown in the table below, various organic additives are added to the bath including 17.3 milliliters per liter of bis(2methoxyethyl) ether, and after plating tests providing a total working of the bath of 4 .amp hours per 1500 milliliter bath, plating results are shown in the table below. Thereafter sodium metabisulfite is added to the bath, also as shown in the table, and plating tests are continued. All platingtests are carried out at 10 amps for 3 minutes and at bath temperatures as shown in the table below.

I TABLE 2 Total amp Current density hrs. per- 1 Total 7 range (a.s.t.)'

1,500 ml. NazSzOs Temp,

bath. g./l. F. pH Volts 400 200 1 0 90 2.91 11.5 4 12- 1s 0 89 3.41 I 18.5 2 17, '17 1 I 90 3.1 18.5

As can be seen from the results in the above table, even after a very short working-period for the bath containing the chromic glycolatecomplex prepared as described hereinabove, noticeable and desirable enhancement in plating performance in the low current density range can be achieved by even a very minor addition of sodium metabisulfite.

We claim:

1. An aqueous electrolytic plating medium for the plating of bright chromium plate, said medium being maintained at a pH within the range of between about 1.5-3.5, andhaving enhanced electrodeposition of said plate, which comprises; I 1 I (A) a complex, water-soluble chromic compound for the deposition of chromium plate and containing nouaromatic carboxylic acid constituent containing less than about 10 carbon atoms and selected from the group consisting of dicarboxylic acids, monocarboxylic acids, monocarboxylic and dicarboxylic acids containing at least one hydroxyl group, and mixtures thereof; and, I I

(B) a sulfite component obtained by providing said medium with at least-onecompound, where such exists, selected from the group consisting of metal sulfites, metal'bisulfites, metal meta bisulfites, trialkyl- .amrnoniumqbisulfites, and mixtures of same, wherein said metal is selected from. the group consisting of alkali metals-and alkaline earth metals.

2. Theplating medium of claim 1 wherein said sulfite component is present in an amount not substantially above about Sweight percent, basis weight of said medium, and said medium has a molar concentration of chromium of at least about 0.1.

3. The plating medium of claim 1 wherein said sulfite component is formed in situ and said alkali metals are selected from the group consisting of sodium and potassium.

4. The plating medium of claim 1 wherein said medium is maintained at a temperature not substantially above about C., and a pH below about 4.9, and said sulfite component is selected from the 'group consisting of metal sulfites, metal meta bisulfites, and mixtures thereof, wherein said metal is selected from the group consisting of alkali metals and alkaline earth metals.

' 5. The plating medium of claim 1 wherein articles to be plated are immersed therein and said complex is present in an amount supplying not substantially above about 75 grams of chromium per liter of the medium.

6. The plating medium of claim 1 wherein said medium additionally contains a salt of a strong acid having a dissociation constant of at least K=10 and a substance selected from the group consisting of boric acid, a substance supplying boric acid equivalent in aqueous solution, and mixtures thereof. 7. The plating medium of claim 6 wherein the cation of said salt is selected from the group consisting of sodium, potassium, and their mixtures, and the anion of said salt is selected from the group consisting of perchlorate, chloride, and mixtures thereof, with said medium containing between about 50-200 grams per liter of said salts;

8. The plating medium of claim 6 wherein said substance supplying boric acid equivalent in aqueous solution is selected from the group consisting of borax, boron oxide, sodiumoxyfluorborate, and mixtures thereof and said medium contains between about 10-70 grams per liter of said substanceL 9. An aqueous electrolytic plating medium for the plating'of bright chromium plate and having enhanced electrodepositionof said plate, which comprises;

.(A). a complex, water-soluble chromic compound containing halogen constituents selected from the group consisting of chloride, fluoride, mixtures thereof and mixtures thereof with other halides, and said chromic ,compound containing carboxylic acid constituents supplied by acids selectedfrom the group consisting of glycolate, lactate, oxalate and mixtures thereof, said medium having a molar concentration of chro- I mium within the range of about 0.5 to about 3; and, (B) a sulfite component obtained by providing said medium with at least one compound, where such exists, selected from the group consisting of metal sulfites, metal bisulfites, metal meta bisulfites, trialkylammonium bisulfites, and mixtures of same, wherein said metal is selected from the group consisting of alkali metals and alkaline earth metals.

10. The plating medium of claim 9 wherein said chromic compound has a molar ratio of chromium atoms to carboxyl constituent within the range of 1:0.7 to 1:3, and the molar ratio of chromium atoms to halogen atoms within the range of 1:0.1 to 1:3.5.

11. The plating medium of claim 9 wherein said bath is maintained within the pH of between about 1.8-4.9 and at a temperature not substantially above about 50 C.

12. An aqueous electrolytic plating medium for the plating of bright chromium plate and having enhanced electrodeposition of said plate, which medium comprises a complex, water-soluble chromic compound for the deposition of chromium plate and containing carboxylic acid constituent, and a salt, where such exists, of formaldehyde bisulfite wherein the anion of said salt is selected from the group consisting of alkali metals and alkaline earth metals.

13. A composition for sustaining plating from an electrolytic plating medium, which medium plates bright chromium plate at a pH of the medium within the range of between about 1.5-4.9, while enhancing the electrodeposition of chromium plate from said medium, which composition consists essentially of a blend of:

(A) a complex, water-soluble chromic plating compound containing non-aromatic carboxylic acid constituent containing less than about 10 carbon atoms and selected from the group consisting of dicarboxylic acids, monocarboxylic acids, monocarboxylic and dicarboxylic acids containing at least one hydroxyl group, and mixtures thereof; and

(B) a sulfite component obtained by providing said medium with at least one compound, where such exists, selected from the group consisting of metal sulfites, metal bisulfites, metal meta bisulfites, trialkylammonium bisulfites, and mixtures of same, wherein said metal is selected from the group consisting of alkali metals and alkaline earth metals.

14. The method of enhancing the electrodeposition of bright chromium plate at low current density from an aqueous chromium plating liquid medium containing a complex, water-soluble chromic plating compound containing non-aromatic carboxylic acid constituent containing less than about 10 carbon atoms and selected from the group consisting of dicarboxylic acids, monocarboxylic acids, monocarboxylic and dicarboxylic acids containing at least one hydroxyl group, and mixtures thereof, which method comprises;

(1) providing said liquid medium with a sulfite component obtained by providing said medium with at least one compound, where such exists, selected from the group consisting of metal sulfites, metal bisulfites, metal meta bisulfites, trialkylammonium bisulfites, and mixtures of same, wherein said metal is selected from the group consisting of alkali metals and alkaline earth metals; and,

(2) passing a current between an anode and a cathode in contact with said liquid medium at low current density.

15. The method of claim 14 wherein said medium is electrolyzed after providing same with sulfite component, and said liquid medium contains not substantially above about 5 weight percent, basis weight of said medium, of said sulfite component.

16. The method of claim 14 wherein said medium is electrolyzed at a current density not substantially above about amperes per square foot.

References Cited UNITED STATES PATENTS 2,693,444 11/1954 Snavely et al. 204-43 3,006,823 10/ 1961 Deyroup 204-51 3,021,267 2/ 1962 Berzins 204--5l 1,922,853 8/ 1933 Kissel 2045 1 F. C. EDMUNDSON, Primary Examiner 

